1. Field of the Invention
This invention generally relates to systems and methods for scanning a beam of light across a specimen. Certain embodiments relate to systems and methods that may include acousto-optical deflectors configured to deflect a beam of light at various angles.
2. Description of the Related Art
Fabricating semiconductor devices such as logic and memory devices may typically include processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes may include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
During each semiconductor device fabrication process, defects such as particulate contamination and pattern defects may be introduced into the semiconductor devices. Such defects may be isolated to a single semiconductor device on a semiconductor wafer containing several hundred semiconductor devices. For example, isolated defects may be caused by random events such as an unexpected increase in particulate contamination in a manufacturing environment or an unexpected increase in contamination in process chemicals which may be used in fabrication of the semiconductor devices. Alternatively, the defects may be repeated in each semiconductor device formed across an entire semiconductor wafer. In an example, repeated defects may be systematically caused by contamination or defects on a reticle. A reticle, or a mask, may be disposed above a semiconductor wafer and may have substantially transparent regions and substantially opaque regions that are arranged in a pattern that may be transferred to a resist. Therefore, contamination or defects on a reticle may also be reproduced in the pattern transferred to the resist and may undesirably affect the features of each semiconductor device formed across an entire semiconductor wafer in subsequent processing.
Defects on semiconductor wafers may typically be monitored manually by visual inspection, particularly in the lithography process because many defects generated during a lithography process may be visible to the naked eye. Such defects may include macro defects which may be caused by faulty processes during this step. Defects which may be visible to the human eye typically have a lateral dimension greater than or equal to approximately 100 xcexcm. Defects having a lateral dimension as small as approximately 10 xcexcm, however, may also be visible on unpatterned regions of a semiconductor wafer. Prior to the commercial availability of automated defect inspection systems such as the systems illustrated in U.S. Pat. Nos. 5,917,588 to Addiego and 6,020,957 to Rosengaus et al., which are incorporated by reference as if fully set forth herein, manual inspection was the most common, and may still be the most dominant, inspection method used by lithography engineers.
A method for manual inspection of a semiconductor wafer may involve placing the semiconductor wafer on a semiautomatic tilt table and rotating the wafer through various angles under a bright light. The semiautomatic tilt table may rotate the semiconductor wafer about a central axis while positioning the semiconductor wafer at different inclinations relative to a plane normal to the central axis. In this manner, an operator may visually inspect the semiconductor wafer for defects as it rotates. The operator may then determine if the defects present on the semiconductor wafer are within an acceptable limit of defects on the semiconductor wafer. An example of a visual inspection method is illustrated in U.S. Pat. No. 5,096,291 to Scott and is incorporated by reference as if fully set forth herein.
Automated inspection systems were developed to decrease the time required to inspect a wafer surface. Such inspection systems may typically include two major components such as an illumination system and a collection-detection system. An illumination system may include a light source such as a laser that may produce a beam of light and an apparatus for focusing and scanning the beam of light. Defects present on the surface may scatter the incident light. A detection system may detect the scattered light and may convert the detected light into electrical signals that may be measured, counted, and displayed on an oscilloscope or other monitor. The detected signals may be analyzed by a computer program to locate and identify defects on the wafer. Examples of such inspection systems are illustrated in U.S. Pat. Nos. 4,391,524 to Steigmeier et al., 4,441,124 to Heebner et al., 4,614,427 to Koizumi et al., 4,889,998 to Hayano et al., and 5,317,380 to Allemand, all of which are incorporated by reference as if fully set forth herein.
Acousto-optical deflection may generally be described as a technique for altering a path of a beam of light that typically involves propagating sound waves through a solid material. Sound waves propagating through the solid material may alter a property such as a refractive index of the solid material. As a result, a beam of light passing through the solid material may be deflected at various angles by the solid material due to the sound waves propagating through the material. In technical applications, acousto-optical deflectors (xe2x80x9cAODsxe2x80x9d), which may also be commonly referred to as acousto-optical scanners, in conjunction with focusing optics, may be used to scan a focused spot of light across a surface of a specimen. Such a technical application may include, for example, inspection of a specimen such as a semiconductor wafer.
An example of a system that includes an AOD is illustrated in U.S. Pat. No. 4,912,487 to Porter et al., which is incorporated by reference as if fully set forth herein. The system includes an argon ion laser beam that may illuminate a specimen surface. An acousto-optical deflector is driven with a chirp signal and placed in the path of the beam to cause it to sweep out raster scan lines. The target is placed on an XY translation stage capable of bi-directional movement. The beam has an angle of incidence normal to the target and the stage moves so that it is scanned along adjacent contiguous strips of equal width. Additional examples of systems that may include AODs are illustrated in U.S. Pat. Nos. 5,633,747 to Nikoonahad, 5,833,710 to Nikoonahad et al., 5,864,394 to Jordan, III et al., and 6,081,325 to Leslie et al., which are incorporated by reference as if fully set forth herein.
Increasing demands for higher throughput and lower cost requirements in semiconductor device manufacturing overall translates into a need for processing and inspection systems having higher accuracy and speed than currently available systems. Such inspection systems may include an AOD. Leading edge AOD scanning inspection systems may include an AOD having a high bandwidth and long acoustic propagation time to provide substantially higher throughput systems with substantially simpler XY translation stages. In addition, such a system may be required to produce substantially uniform spot sizes and substantially uniform brightness across a scan line for substantially constant sensitivity throughout the scan. If a sensitivity of such a system is not consistent across the scan line, system-to-system matching as in multiple machine system applications and environments may be problematic. In addition, producing substantially constant spot sizes across larger scan lengths may improve a data acquisition rate of a system because a larger portion of a specimen may be scanned in a single scan. In this manner, a throughput of such a system may also be increased.
In an embodiment, a system may be configured to scan a focused spot of light over a surface of a specimen. The system may include an AOD and optics configured to focus a beam of light to a small spot and to scan this spot across a line considerably longer than the size of the spot. The AOD may be operated in xe2x80x9cdeflection mode,xe2x80x9d in which the entire AOD is filled with a nearly constant frequency sound wave, which deflects the beam at a nearly constant angle. In this mode, a scan line may be produced by varying the AOD frequency as a function of time. Alternatively, the AOD may be operated in xe2x80x9cchirp mode,xe2x80x9d in which a portion of the AOD is filled with a sound wave with rapidly varying frequency (xe2x80x9cchirp packetxe2x80x9d), which focuses the beam to a small spot. In this mode, a scan line may be produced by the propagation of the chirp packet across the length of the AOD.
The system may also include a relay lens. The relay lens may be configured to collimate the light from a scan line produced by an AOD operated in chirp mode. The optical axis of the relay lens may be substantially centered on the scan line produced by the AOD. The optical axis of the relay lens may also be substantially perpendicular to the scan line produced by the AOD but not parallel to the chief ray produced by the AOD. In addition, the system may include an objective lens. The optical axis of the objective lens may be substantially parallel to but substantially de-centered with respect to the optical axis of the relay lens. The collimated light, however, may be substantially centered on the objective lens.
The system may also include a prism or mirror assembly located between the relay lens and an objective lens. The prism or mirror assembly may be configured to re-center the collimated light onto the objective lens to avoid the need to de-center the objective lens from the axis of the relay lens.
The objective lens may be configured to focus the collimated light to a focal plane. The objective lens may be oriented substantially parallel to the focal plane. In addition, the optical axis of the objective lens may be substantially centered on and perpendicular to the focal plane. The focal plane may be substantially parallel to the surface of the specimen. Therefore, such a system may reduce, and may even substantially eliminate, field tilt of the system. Field tilt may be generally described as an angle at which a focal plane of a system may be located with respect to a surface of a specimen. Such field tilt may result, for example, from using an AOD in chirp mode with centered relay optics.
Field tilt may not be problematic for field sizes that are not large relative to the spot size of light within the field. Relatively small field sizes, however, may have several disadvantages. For example, a system that may have a relatively small field size may have a relatively low throughput and may require a complex, high performance XY translation stage in comparison to a system that may have a relatively large field size. As field size increases, however, field tilt may reduce spot size uniformity across the field. For example, spot sizes further from the center of the field may become larger and defocused due to tilt of the focal plane. For systems with field tilt, the amount of defocus scales as the square of the distance from the center of the scan line. In this manner, sensitivity of such a system may also vary across the field. In addition, if the sensitivity of the system varies, then the performance of a plurality of such systems may vary from system-to-system. Therefore, field tilt may become more problematic in leading edge inspection systems.
Because field tilt of a system as described herein may be reduced, and even substantially eliminated, the spot size of a beam of light on the surface of the specimen may be substantially independent of a position of the beam of light on the surface of the specimen. Therefore, such a system may have a relatively large field size. Consequently, the system may also have a relatively high throughput and a simple and relatively inexpensive XY translation stage. In addition, spots throughout substantially the entire field of such a system may have a substantially constant size and focus. Therefore, the sensitivity of the system may be substantially independent of the position of the beam of light on the surface of the specimen. In this manner, because the sensitivity of the system may be substantially uniform, performance of a plurality of such systems may be substantially uniform from system-to-system, thereby enabling improved system-to-system matching.
In an embodiment, a system may be configured to scan a beam of light over a surface of a specimen. The system may include a first AOD and a second AOD. The first AOD may be configured to direct the beam of light at various angles through an optical system onto the second AOD. The brightness of the beam produced by the first AOD may be calibrated with a substantially uniform scattering feature.
The first AOD may be operated in deflection mode, where the drive signal duration is longer than the propagation time of an acoustic wave across the light beam. The system may also include a lens configured to expand the beam created by the first AOD and convert the angular scan into a substantially parallel scan. The second AOD may be operated in chirp mode, where the drive signal duration is approximately equal to the propagation time of the acoustic wave across the beam. The second AOD may be configured to function as a traveling lens to focus the scanning beam. The length of a chirp packet traveling in the second AOD may be much smaller than the length of the second AOD. In this manner, light directed by the second AOD may scan a surface of a specimen.
The amplitude of the drive signal applied to the first AOD may be modulated to control the brightness of the deflected beam. This intensity modulation may be used to compensate for transmission losses over the length of the second AOD or over the entire inspection system. xe2x80x9cTransmission lossxe2x80x9d of an AOD may generally describe changes in the intensity of the light deflected by the AOD. The transmission loss is caused by attenuation of the acoustic chirp packet as the chirp packet propagates through the solid medium of the AOD. A xe2x80x9cchirp packetxe2x80x9d may generally refer to an acoustic wave propagating through an AOD produced by an excitation such as a radio-frequency signal from a generator coupled to the AOD through a transducer. Transmission losses may cause changes in intensity of light deflected by the AOD over a length of the AOD. In addition, an intensity of the deflected light may become less uniform as a length of an AOD increases.
In an embodiment, the amplitude of a drive signal applied to the first AOD may be modulated such that an intensity of the light over the scan line of the first AOD may increase as transmission losses over a length of the second AOD increase. Therefore, such a system may compensate for acoustical attenuation over the length of the second AOD. This mechanism may also be used to compensate for other losses in the optical system. In this manner, an intensity of the light directed by the second AOD may be substantially independent of a position of the directed beam of light on the surface of the specimen. As such, a sensitivity of the system may be substantially independent of a position of the directed beam of light on the surface of the specimen. Therefore, as described above, a performance of a plurality of such systems may be substantially uniform from system-to-system, thereby enabling system-to-system matching.
In an additional embodiment, a system may be configured to scan a beam of light over a surface of a specimen. The system may include an AOD operating in chirp mode. The AOD may include more than one chirp packet at the same time. While a first chirp packet is propagating through the AOD and is illuminated to form a scan line, a second chirp packet may be prepared to begin a subsequent scan line. This arrangement, called xe2x80x9cprefilling,xe2x80x9d may eliminate the lost time associated with filling the chirp packet at the beginning of each scan line.
In addition, the system may include a field stop. The field stop may be configured to allow only one chirp packet at a time to scan the specimen. For example, while the first chirp packet is scanning the specimen, the field stop may block light deflected from the second chirp packet; conversely, when the second chirp packet is fully prepared and begins scanning the specimen, the field stop then blocks light from the first chirp packet.
An average data rate of such a system may be approximately equal to a peak data rate of the system. By pre-filling the AOD in this manner, the time required to fill an acoustic cell may be substantially eliminated from the process time of the system. In addition, such a system may substantially continuously scan light over a surface of a specimen. For example, a process time of such a system may include only a time required to scan the specimen, a fill time of a prescanner AOD, and a reset time of the electronics. As such, a data rate of such a system may only be limited by a fill time of the pre-scanner and a reset time of the electronics. In addition, because a throughput of such a system may depend on the data rate of the system, a throughput of such a system may also be increased.